This invention relates to a tensioner assembly for adjusting the tension of a belt for driving vehicle engine accessories such as an alternator, a water pump and the compressor of an air-conditioner.
This tensioner assembly (tension adjusting device) T is used in most belt transmission systems for transmitting the rotation of an engine crankshaft to various vehicle engine accessories, as shown in FIG. 4.
The tensioner assembly T includes a bracket (also known as a “pulley arm”) 3 provided on the slack side of the belt 1 so as to be pivotable about a pivot shaft 2, a tension pulley 4 supported on the pulley arm 3 at the remote end from the pivot shaft 2, and a tensioner A which applies an adjusting force to the pulley arm 3, thereby pressing the tension pulley 4 against the belt 1, keeping constant the tension of the belt 1.
The pulley arm 3 is formed by casting e.g. an aluminum alloy or iron as a monolithic body. As shown in FIGS. 3A and 4, the pulley arm 3 includes a pivotally supported section 10 through which the pivot shaft 2, which is supported on a stationary member 5 such as a cylinder block, is inserted, a pulley supporting section 20 on which the tension pulley 4 is rotatably supported, and a damper mounting section 30 to which the tensioner A is connected. Many of such tensioners A are hydraulic tensioners which perform their damper, functions using hydraulic pressure.
The pivotally supported section 10 is formed with a through hole 11 through which a support bolt 13 as the pivot shaft 2 is inserted with a liner 12 disposed between the bolt 13 and the hole 11. The pulley supporting section 20 and the damper mounting section 30 are thus pivotable about the pivotally supported section 10, more specifically about the center axis of the pivot shaft 2.
The tension pulley 4 is mounted on a base portion 21 of the pulley supporting section 20 through a rolling bearing 25 such as a ball bearing. The base portion 21 is formed with a through hole 21a extending in the axial direction of the tension pulley 4. The through hole 21a has a cylindrical surface having a diameter large enough such that the shank of a support bolt 23 can be inserted through the hole 21a. The rolling bearing 25 has its inner race 25a fitted on the outer surface 21b of the base portion 21.
The rolling bearing 25 further includes an outer race 25d disposed around the inner race 25a through a plurality of rolling elements 25c circumferentially retained by a retainer 25b. The tension pulley 4 is fixedly press-fitted on the outer race 25d. 
The rolling bearing 25 is pushed by the support bolt 23 and a nut 24 thorough a washer 22 from the axial front side (from which the rolling bearing 25 and the tension pulley 4 are fitted onto the base portion 21) toward the axial rear side until the inner race 25a abuts an end surface 21c forming a shoulder of the base portion 21 at the axial rear portion of the base portion 21 such that the rolling bearing 25 is fixed to the base portion 21. The tension pulley 4 is thus rotatably supported by the pulley arm 3 such that the tension pulley 4 guides the belt 1.
FIG. 3B shows a different arrangement in which the rolling bearing 25 is fixed to the pulley arm 3 only by the support bolt 23 and without using the nut 24. In this arrangement, an internal thread 21d is formed on the inner surface of the through hole 21a over the entire length thereof. By inserting the shank 23b of the support bolt 23 toward the axial rear side with the head 23a of the bolt 23 positioned on the axial front side, the internal thread 21d of the through hole 21a is brought into threaded engagement with (meshes with) the external thread 23d on the support bolt 23 (as shown in European patent publication 1783400).
In the arrangement of FIG. 3A, the rolling bearing 25 is fixed to the base portion 21 by tightening the support bolt 23 and the nut 24 from both axial ends of the base portion 21. This is troublesome.
In the arrangement of FIG. 3B, since no nut 24 is used, the rolling bearing 25 can be fixed to the base portion 21 simply by tightening the support bolt 23 from the axially front end.
But in the arrangement of FIG. 3B, if the support bolt 23, which has been inserted into the through hole 21a of the base portion 21, is tightened with an excessive torque, cracks may form at the corner 21f of the base portion 21. If a force exceeding the projected upper limit continuously acts on the pulley arm 3 too, cracks may form at the corner 21f of the base portion 21.
Such cracks form because the portion of the base portion 21 supporting the rolling bearing 25 has an especially small wall thickness t, so that stress tends to concentrate on the corner 21f between the outer peripheral surface 21b of the thin-walled portion of the base portion 21 and the end surface 21c of the shoulder of the base portion 21 when the support bolt 23 is tightened or when force is applied from the pulley arm 3.